1. Field of the Invention
The present invention relates to a method for forming a groove and a method for manufacturing an optical waveguide element, and more specifically to a method for forming a groove that enable exact formation of vertical wall surface and a method for manufacturing an optical waveguide element applying this forming method.
2. Prior Art
An optical waveguide element formed by covering a large refractive index core layer with a small refractive index clad layer to transmit an optical signal has been used practically as various optical communication parts.
FIG. 1A and FIG. 1B show a waveguide type optical switch having a groove formed at crossing of a plurality of waveguides each of which is a kind of said optical waveguide element. In FIG. 1A, numeral 1 denotes an input waveguide into which an optical signal is entered, numerals 2 and 3 denote output waveguides which are branched at angle of 90 degrees each other from the input waveguide 1, and numeral 4 denotes a groove which is formed at 45 degrees to orthogonal crossing potion of the output waveguides 2 and 3.
As shown in FIG. 1B, the input and output waveguides 1, 2 and 3 comprising a core layer 7 which is quadrangular in cross section and covered upper and lower portions by the over clad layer 5 and the under clad layer 6. Further, the groove 4 is formed vertically from the over clad layer 5 through the core layer 7.
The above explained optical switch emits an optical signal from output waveguide 3 or 2 by thermally evaporating or condensing liquid which has refractive index of almost the same as the core layer 7 and is filled in the groove 4, thereby high speed switching function which changes propagating direction of an optical signal is provided, and is highly evaluated as switch parts of optical communication. Details of such optical switch are disclosed in Japanese laid-open patent number 04-146402.
FIG. 2 shows forming method of the groove 4 which is an important component of said optical switch.
In step (a), a composite material comprising the core layer 7, the over clad layer 5 covering upper of the core layer 7 and the under clad layer 6 covering lower of the core layer 7 is prepared.
In step (b), a metal film 8 such as Cr film is formed on the over clad layer 5 by electron beam evaporating or spattering.
In step (c), a photo resist film 9 is formed over the metal film 8, thereafter a resist film groove pattern 10 is formed according to a groove pattern to be formed by performing exposure and development.
In step (d), a metal filmgroovepattern 11 is formed by etching processing masked with the resist film 9 and successive removing treatment of the resist film 9.
In step (e), the groove 4 is formed by dry etching processing such as reactive ion etching (RIE) masked with the metal film 8.
Finally, in step (f), the optical switch as shown in FIG. 1A and B is completed by removing the metal film 8.
The method for forming a groove using dry etching processing as explained above is suitable for formation of a groove of an optical switch that fine width dimension such as ten and several μm or several μm order is required. Further, this method is also applicable, for example, to formation of a groove for filling temperature compensating material in temperature nondependent optical multiplexer/demultiplexer disclosed in Japanese laid-open patent number 2001-051138.
However, according to conventional method for forming a groove, verticality of wall surface in a groove become frequently imperfect, thereby, for example, in case of an optical switch, problem that reflecting loss is increased by optical axis difference of reflecting light caused in the groove 4 is arisen.
To explain the cause of this problem, the movement of the etching gas 12 at dry etching processing is shown in FIG. 3. Namely, the gas 13 containing plenty of movement component to horizontal direction and being contained in the etching gas 12 acts to form a taper portion 14 on upper wall surface in the groove 4, if this reaches to core layer 7, axis difference of reflected light is caused by the taper portion 7a to lower the ability of an optical switch.
Influence of insufficient verticality of wall surface in a groove is a problem which occurs not only in an optical switch but also in a temperature compensating material of temperature nondependent optical multiplexer/demultiplexer, insufficient verticality acts to lower ability of an optical multiplexer/demultiplexer as optical parts.